ECG-Myocardial Infarction

Source: ABC of ECG (BMJ)


Hyperacute T waves
The earliest signs of acute myocardial infarction are subtle
and include increased T wave amplitude over the affected area.
T waves become more prominent, symmetrical, and pointed
(“hyperacute”). Hyperacute T waves are most evident in the
anterior chest leads and are more readily visible when an old
electrocardiogram is available for comparison. These changes
in T waves are usually present for only five to 30 minutes after
the onset of the infarction and are followed by ST segment
changes.


ST segment changes
In practice, ST segment elevation is often the earliest recognised
sign of acute myocardial infarction and is usually evident within
hours of the onset of symptoms. Initially the ST segment may
straighten, with loss of the ST­T wave angle . Then the T wave
becomes broad and the ST segment elevates, losing its normal
concavity. As further elevation occurs, the ST segment tends to
become convex upwards. The degree of ST segment elevation
varies between subtle changes of < 1 mm to gross elevation of > 10 mm.



Pathological Q waves
As the acute myocardial infarction evolves, changes to the QRS
complex include loss of R wave height and the development of
pathological Q waves.
Both of these changes develop as a result of the loss of
viable myocardium beneath the recording electrode, and the
Q waves are the only firm electrocardiographic evidence of
myocardial necrosis. Q waves may develop within one to two
hours of the onset of symptoms of acute myocardial infarction,
though often they take 12 hours and occasionally up to 24
hours to appear. The presence of pathological Q waves,
however, does not necessarily indicate a completed infarct. If
ST segment elevation and Q waves are evident on the
electrocardiogram and the chest pain is of recent onset, the
patient may still benefit from thrombolysis or direct
intervention.
When there is extensive myocardial infarction, Q waves act
as a permanent marker of necrosis. With more localised
infarction the scar tissue may contract during the healing
process, reducing the size of the electrically inert area and
causing the disappearance of the Q waves.



Resolution of changes in ST segment
and T waves
As the infarct evolves, the ST segment elevation diminishes and
the T waves begin to invert. The ST segment elevation
associated with an inferior myocardial infarction may take up to
two weeks to resolve. ST segment elevation associated with
anterior myocardial infarction may persist for even longer, and
if a left ventricular aneurysm develops it may persist indefinitely.
T wave inversion may also persist for many months and
occasionally remains as a permanent sign of infarction.



Reciprocal ST segment depression

ST segment depression in leads remote from the site of an
acute infarct is known as reciprocal change and is a highly
sensitive indicator of acute myocardial infarction. Reciprocal
changes are seen in up to 70% of inferior and 30% of anterior
infarctions.
Typically, the depressed ST segments tend to be horizontal
or downsloping. The presence of reciprocal change is
particularly useful when there is doubt about the clinical
significance of ST segment elevation.

Reciprocal change strongly indicates acute infarction, with a
sensitivity and positive predictive value of over 90%, though its
absence does not rule out the diagnosis.
The pathogenesis of reciprocal change is uncertain.
Reciprocal changes are most frequently seen when the infarct is
large, and they may reflect an extension of the infarct or occur
as a result of coexisting remote ischaemia. Alternatively, it may
be a benign electrical phenomenon. The positive potentials that
are recorded by electrodes facing the area of acute injury are
projected as negative deflections in leads opposite the injured
area, thus producing a “mirror image” change. Extensive
reciprocal ST segment depression in remote regions often
indicates widespread arterial disease and consequently carries
a worse prognosis.